1. Field of the Invention
The present invention relates to an ATV (All Terrain Vehicle: an uneven terrain traversing, mounted type vehicle, also called four-wheeled buggy car) drive shaft and to an undercut free type constant velocity joint disposed on the outboard side of a drive shaft for a vehicle having no power steering.
2. Detailed Description of the Prior Art
As is known in the art, in automobiles having no power steering and various vehicles similar thereto, constant velocity joints capable of transmitting rotational power at constant velocity even when there is an angular or axial displacement between two shafts have been installed in a power transmission path for transmission of drive power from the engine to wheels. As an example of a vehicle having such power transmission path, a so-called ATV which is an uneven terrain traversing, four-wheeled or three-wheeled, mounted type vehicle, equipped with balloon tires and designed to freely traverse wastelands, sandy beaches and the like has come to be widely known.
The power transmission device for these various vehicles will be explained with said ATV used as a representative example. As conceptually shown in FIG. 6, the power from an engine 21 is outputted from the output shafts on the front and rear sides via a speed change mechanism in the interior and is inputted to differential gears 24 and 25 on the front and rear sides via power transmission means 22 and 23, such as chains or propeller shafts. And, the engine power inputted to the differential gears 24 and 25 is reduced in speed by the mechanism of the differential gears 24 and 25 and is converted to a rotational power orthogonal thereto, whereupon it is transmitted to the wheels 28 and 29 through left and right drive shafts 26 and 27. In the example shown in the same figure, constant velocity joints are used for joints A between the drive shaft 26 on the front side and the differential gear 24 and for joints B in the wheels 28. In addition, there are cases where constant velocity joints are used for joints C between the drive shaft 27 on the rear side and the differential gear 25 and for joints D in the wheels 29. Further, when propeller shafts are used as the power transmission means 22 and 23, there are cases where constant velocity joints are used for joints E and F between the propeller shafts and the output shafts of the engine (speed change mechanism) 21 and joints G and H in the differential gears 24 and 25.
FIG. 7 shows the drive shaft 26 on the front side. In order to allow the drive shaft 26 to make angular displacement and axial displacement following the movement of the wheel 28 during cornering, traversing uneven terrains or the like movement, a slide type constant velocity joint (a constant velocity joint allowing angular displacement and axial displacement between two shafts) 30 and a fixed type constant velocity joint (a constant velocity joint allowing angular displacement between two shafts) 31 are used in pair for joining the drive shaft 26. In the example shown in the same figure, one end (the inboard side) of the drive shaft 26 is joined to the differential gear 24 (at the joint A) through the slide type constant velocity joint (a double offset type constant velocity joint, hereinafter referred to as “DOJ”) 30, while the other end (the outboard side) of the drive shaft 26 is joined to the wheel 28 (at the joint B) through a fixed type constant velocity joint (Rzeppa type constant velocity joint: ball fixed joint, hereinafter referred to as “BJ”) 31.
Heretofore, as said DOJ and BJ for vehicles such as ATVs, those for passenger cars have been frequently converted to be used as such. And, in vehicles small in size and narrow in width, such as ATVs, it is suitable to use a drive shaft which is light-weight and compact and which has satisfactory operability. To meet such demand, Japanese Patent Laid-Open 2001-97063 which follows, for example, discloses the use of a double offset type constant velocity joint (DOJ) on the inboard side and an undercut free type constant velocity joint (hereinafter referred to as “UJ”) on the outboard side, in a drive shaft for transmitting drive power to wheels through constant velocity joints on the inboard and outboard sides.
The UJ disposed on the outboard side of this drive shaft, basically, as shown in FIG. 8, comprises an outer joint member 12 with a spherical inner peripheral surface 12a formed with a plurality of track grooves 13, an inner joint member 14 with a spherical outer peripheral surface 14a formed with a plurality of track grooves 15, a plurality of torque transmitting balls 16 disposed in ball tracks formed by the opposed track grooves 13 and 15 of both joint members 12 and 14, and a cage 18 interposed between both joint members 12 and 14 and formed with a plurality of pockets 17 holding the torque transmitting balls 16.
And, it is stated in said Document that the track offset angle of the UJ is set at 5°-7° and that the cage offset amount of the UJ is set at 0, and that with a conventional UJ, the cage offset angle has been set at not less than 1°. That is, in this UJ, as shown in FIG. 8, the centers Oaa and Obb of track grooves 13 and 15 of the outer and inner joint members 12 and 14 are offset by an equal distance Laa to axially opposite sides with the joint center O used as a reference. A track offset angle φaa (an offset angle at the track grooves 13 and 15) consisting of ∠0aaQO (or ∠ObbQO) is set at 5°-7°. In this case, since the cage offset amount is 0, the Lcc in the figure is 0. Further, in the conventional UJs, the centers Occ and Odd of the inner and outer spherical surfaces 18a and 18b of the cage 18, respectively, are offset by an equal distance to the axially opposite sides with the joint center O used as a reference. A cage offset angle φcc (the offset angle of each of the spherical surfaces 18a and 18b of the cage 18) consisting of ∠OddQO (or ∠OccQO) is set at not less than 1°. In addition, in the following description, the sum of the track offset angle and the cage offset angle φcc will be referred to as the total offset angle (φaa shown in FIG. 8). Therefore, the track offset amount in FIG. 8 is an amount (Laa−Lcc) obtained by subtracting the cage offset amount Lcc from the total offset amount Laa.
Further, in a conventional UJ, in order to improve the function of a cage 18 to guide torque transmitting balls 16 and from the standpoint of attaching importance to the securement of constant velocity nature of the joint, the axial clearance δ between a pocket 17 of the cage 18 and the torque transmitting ball 16, that is, the value δ obtained by subtracting the diameter of the torque transmitting ball 16 from the axial width of the pocket 17 is set so that δ<0 (negative clearance) to give an interference between the pocket 17 and the torque transmitting ball 16, which has been common practice. In this case, when the axial clearance δ is small (excessively negative), the torque transmitting balls 16 can hardly roll on the ball tracks and in the pockets 17, a fact which is disadvantageous from the standpoint of rotational resistance when this type of joint transmits torque while taking a working angle; thus, adversely affecting steerability. In contrast, when the axial clearance δ is made large (positive clearance is made excessively large), the rollability of the torque transmitting balls 16 is enhanced to reduce the rotational resistance, but the guiding function of the cage 18 for the torque transmitting balls 16 is impaired to break constant velocity nature of this type of joint, leading to abnormal sound and a feeling of being caught during steering, and hence to a deterioration in steering feeling. In addition, Japanese Patent Laid-Open Nos. 2000-266071 and 2002-5186 specify the axial pocket clearance δ of a constant velocity joint installed in general automobiles including passenger cars, not a constant velocity joint installed in vehicles having no power steering represented by ATVs which are the subject of the invention.
Since car body weight restrictions for ATVs are severe, further weight reduction and size compaction are required of drive shafts therefor. Further, ATVs are small in size, narrow in width and high in height, so that constant velocity joint installed in the drive shaft has its normal working angle reaching as much as about twice that in passenger cars. For this reason, with passenger car specifications, there may be the danger of the operating stability of the constant velocity joint being impaired depending on usage conditions and the like. Further, with ATV constant velocity joints, about ½ of the durability (life) of passenger cars and the like is sufficient as considered from balance between market performance and the term of guarantee. Therefore, as considered on the basis of passenger car specifications as they are, there is a feeling of excessive quality. As to the rpm used, about half for passenger car specifications is sufficient as considered from balance with vehicle speed, and the same may be said. On the other hand, the same degree of strength including twist strength as that for passenger car specifications is required.